Polymer electrolyte membrane fuel cell assemblies are relatively low temperature low operating pressure fuel cell assemblies that utilize a catalyzed polymer membrane electrolyte to process air and a hydrogen-rich fuel to produce electricity and water. PEM fuel cells are well suited for use in mobile applications such as automobiles, buses, and the like, because they are relatively compact, light in weight and operate at essentially ambient pressure. This type of fuel cell system can be cooled evaporatively by a non-circulating water coolant. The cooler has an outer plate that is formed with channels which contain the water coolant. The cooler also has an inner porous plate which faces the cathode side of the fuel cell through which an air reactant stream flows. The cell is cooled by water which flows through the porous plate to the air stream and evaporates therein so as to cool the cell. During operation, a small amount of air also diffuses through the porous plate into the water coolant. The cathode reactant stream effluent will comprise a water vapor and air mixture. The water vapor and air components of the cathode effluent mixture are passed through a condenser where water is condensed out of the mixture. The resultant water/air mixture is then passed through a separator station where the condensed water is removed from the mixture and air is vented out of the fuel cell assembly. The water is then returned to the coolant flow field in the fuel cell assembly.
The separator air venting portion of the system typically includes a passage to ambient surroundings which passage permits controlled air flow from the separator by means of mechanical valves and/or fixed nozzles. These valves and/or fixed nozzles in the air bleed passage serve to control back pressure in the liquid/air separator during normal operation. The air stream which is vented from the separator is humid after leaving the condenser. This fact causes operational problems during freezing conditions since the valves and/or nozzles in the air bleed line can freeze up so that air flow can no longer be properly controlled from the system, thus forcing shut down of the system and of the power plant. This problem can be fixed by heating the air bleed line but this solution requires additional heating equipment in the system that increases system complexity and cost.
It would be highly desirable to have a back pressure and flow control system for venting air from the separator which control system can be used in freezing conditions and does not require the use of any complicated mechanical devices during operation of the fuel cell power plant.